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The extinct group of the Pycnodontiformes is one of the most characteristic components of the Mesozoic and early Cenozoic fish faunas. These ray-finned fishes, which underwent an explosive morphological diversification during the Late Cretaceous, are generally regarded as typical shell-crushers. Here we report unusual cutting-type dentitions from the Paleogene of Morocco which are assigned to a new genus of highly specialized pycnodont fish. This peculiar taxon represents the last member of a new, previously undetected 40-million-year lineage (Serrasalmimidae fam. nov., including two other new genera and Polygyrodus White, 1927) ranging back to the early Late Cretaceous and leading to exclusively carnivorous predatory forms, unique and unexpected among pycnodonts. Our discovery indicates that latest Cretaceous–earliest Paleogene pycnodonts occupied more diverse trophic niches than previously thought, taking advantage of the apparition of new prey types in the changing marine ecosystems of this time interval. The evolutionary sequence of trophic specialization characterizing this new group of pycnodontiforms is strikingly similar to that observed within serrasalmid characiforms, from seed- and fruit-eating pacus to flesh-eating piranhas.

The collapse of marine ecosystems during the end-Cretaceous mass extinction involved the base of the food chain [1] up to ubiquitous vertebrate apex predators [2-5]. Large marine reptiles became suddenly extinct at the Cretaceous-Paleogene (K/Pg) boundary, whereas other contemporaneous groups such as bothremydid turtles or dyrosaurid crocodylomorphs, although affected at the familial, genus, or species level, survived into post-crisis environments of the Paleocene [5-9] and could have found refuge in freshwater habitats [10-12]. A recent hypothesis proposes that the extinction of plesiosaurians and mosasaurids could have been caused by an important drop in sea level [13]. Mosasaurids are unusually diverse and locally abundant in the Maastrichtian phosphatic deposits of Morocco, and with large sharks and one species of elasmosaurid plesiosaurian recognized so far, contribute to an overabundance of apex predators [3, 7, 14, 15]. For this reason, high local diversity of marine reptiles exhibiting different body masses and a wealth of tooth morphologies hints at complex trophic interactions within this latest Cretaceous marine ecosystem. Using calcium isotopes, we investigated the trophic structure of this extinct assemblage. Our results are consistent with a calcium isotope pattern observed in modern marine ecosystems and show that plesiosaurians and mosasaurids indiscriminately fall in the tertiary piscivore group. This suggests that marine reptile apex predators relied onto a single dietary calcium source, compatible with the vulnerable wasp-waist food webs of the modern world [16]. This inferred peculiar ecosystem structure may help explain plesiosaurian and mosasaurid extinction following the end-Cretaceous biological crisis.

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Mosasaurids (Mosasauridae) were specialized marine lizards that evolved and radiated in the Late Cretaceous. Their diversity peaked in the Maastrichtian, with the most diverse faunas known from Morocco. Here we describe a new species of mosasaurid from this fauna. Pluridens serpentis sp. nov. is described based on two complete skulls and referred jaws. It is referred to Pluridens based on the elongate and robust jaws, small teeth, and specialized tooth implantation. Pluridens is referred to Halisaurinae based on the posteriorly expanded premaxilla, long premaxillamaxilla suture, broad premaxillary facet on the maxilla, closed otic notch, and small, striated, hooked teeth. The orbits are reduced relative to other halisaurines while the snout is robust and flat with a broad, rounded tip. The jaws bear numerous small, hooked, snake-like teeth. Skulls imply lengths of 5-6 meters; referred material suggests lengths of ≥10 meters. Pluridens' specialized morphology -especially the contrasting large size and small teeth -suggests a distinct feeding strategy. Small orbits imply that P. serpentis relied on nonvisual cues including touch and chemoreception during foraging, as in modern marine snakes. Numerous neurovascular foramina on the premaxillae are consistent with this idea.The small teeth suggest proportionately small prey. The dentary becomes massive and robust in the largest individuals, suggesting sexual selection and perhaps sexual dimorphism, with the mandibles possibly functioning for combat as in modern beaked whales and lizards. The new mosasaur emphasizes how Maastrichtian mosasaurids were characterized by high species richness, functional diversity of niches occupied, and a certain degree of endemism, i.e. geographic specialization. and continued diversifying until the end of the Cretaceous, just prior to the K-Pg extinction.

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